This invention relates to an exposure method and, more particularly, to a lithographic exposure method suitable for transferring an integrated-circuit pattern of a mask onto a semiconductor wafer having a radiation-sensitive coating, during the manufacture of semiconductor devices.
In conventional lithographic exposure for transferring, onto a wafer, a fine pattern of a photomask (which hereinafter will be referred to simply as "mask") for the manufacture of semiconductor devices or the like, the mask pattern is transferred onto the wafer only by a high-resolution exposure. That is, before or after such high-resolution exposure, no irradiation of a beam to which the radiation-sensitive layer on the wafer is sensitive is effected, for the sake of the pattern transfer.
It is known in the art that, for most radiation-sensitive materials for the radiation-sensitive layer on the wafer, the pattern transfer requires irradiation of the sensitizing beam of such an amount that is not less than a certain amount D.sub.0 (threshold). Namely, if the amount of irradiation is less than the threshold D.sub.0, there remains, after development, a layer thickness which is the same as the layer thickness that would remain where no irradiation or exposure is effected. Therefore, the irradiation of the sensitizing beam has to be continued until the amount of irradiation reaches the threshold D.sub.0. As the result, a longer time is required for the exposure because the intensity is relatively low in the high-resolution exposure such as for the manufacture of semiconductor integrated circuits.
The term "beam" is referred to in this Specification to imply various kinds of beams such as ultraviolet rays, deep ultraviolet rays, X-rays, charged-particle rays, laser beams and so on. Among these beams, particularly the X-rays, when they are used in the exposure, require a very long exposure time. This is because the X-rays are divergent rays so that, in the X-ray exposure, the mask is located at a position far remote from an X-ray source in order to improve the degree of parallelism of the X-rays impinging on the mask. Alternatively, an X-ray collimator is used to extract only parallel components out of the diverging X-rays. In any case, the intensity is very low in the case of X-ray exposure, which necessarily requires a prolonged exposure time.